In certain applications for microwave transmission line couplings, it is desirable to provide a seal so that coaxial and other microwave components can be suitably isolated from ambient environments which include air, water, pressure changes and chemicals. It is also important that the center conductor in coaxial couplings be firmly held in position despite axial stresses that may be encountered due to temperature variations, mechanical stresses, or during installation of coaxial cables connected to the couplings. The center conductor is held in place with dielectric elements that seat against axial surfaces on both the center and outer conductors, see for example U.S. Pat. No. 4,596.435.
Various dielectric materials have been proposed for use in coaxial couplings including polytetrafluoroethlene (PTFE) sold under the trademark TEFLON (by the DuPont Company). PTFE, which is an excellent dielectric material for use in coaxial couplings, however, does not always form a satisfactory seal. PTFE has been used to coat metal surfaces for cooking utensils, see for example, U.S. Pat. Nos. 3,243,321; 3,697,309 and 4,123,401. These processes involve the application of a PTFE dispersion to a prepared metal surface which is then baked at an elevated temperature. Although these techniques provide a thin PTFE surface that adheres to metal, it is not satisfactory for a microwave transmission line coupling. For example, a relatively thick PTFE insulator is required between the central and outer conductors of a coaxial connector.